Leptospirosis is a bacterial infection caused by spiral-shaped bacteria of the genus Leptospira, spread to humans mainly through contact with water or soil contaminated by the urine of infected animals. It is considered the most widespread zoonotic disease in the world, capable of causing anything from a mild flu-like illness to life-threatening organ failure. The infection is especially common in tropical regions and after flooding events, though cases occur on every continent except Antarctica.
How People Get Infected
The bacteria live in the kidneys of carrier animals and are shed in urine, sometimes for months or years. Wild rodents, especially the brown rat, are the most important reservoir. Rats typically carry the infection without showing any symptoms, and studies using different inoculation routes in rats have confirmed that mucosal exposure (through the eyes, nose, or mouth) is an efficient path to kidney colonization and ongoing bacterial shedding.1PLOS Neglected Tropical Diseases. Comparison of Mucosal, Subcutaneous and Intraperitoneal Routes of Rat Leptospira Infection Research in Brazilian urban slum communities has estimated that a rat population of just 82 individuals can shed roughly 91 billion leptospires into the environment per day, with high bacterial density in the soil surrounding households.2PLOS Neglected Tropical Diseases. Patterns in Leptospira Shedding in Norway Rats (Rattus norvegicus) from Brazilian Slum Communities at High Risk of Disease Transmission
For humans, infection happens when the bacteria enter through small cuts, abrasions, or the mucous membranes of the eyes, nose, or mouth. Swallowing or wading through contaminated floodwater is one of the most common routes. You can also catch it through direct contact with infected animal urine, blood, or tissue. Dogs, cattle, pigs, and many wildlife species can all carry the organism in their renal tubules and shed it in urine.3PubMed Central. Leptospirosis in humans Occupational exposure matters too: farmers, sewer workers, veterinarians, and military personnel training in tropical environments all face higher risk.
Early Symptoms
The incubation period usually runs between two and 30 days, with most people developing symptoms around 7 to 12 days after exposure. The early, or “leptospiremic,” phase looks a lot like many other tropical infections, which is part of why leptospirosis is so often misdiagnosed. Typical symptoms include sudden high fever, intense headache, chills, muscle pain (especially in the calves and lower back), nausea, vomiting, and diarrhea. Some patients also develop a distinctive redness of the eyes known as conjunctival suffusion, where the whites of both eyes become bloodshot without the pus or discharge you would see in a typical eye infection.4PubMed Central. Bilateral conjunctival suffusion: An ocular manifestation of leptospirosis
This early phase usually lasts about a week before the fever breaks. Many people recover at this stage and never progress further. But in a subset of patients, a second, more dangerous phase follows after a brief period of apparent improvement.
When It Turns Dangerous
Roughly 5 to 15 percent of symptomatic infections progress to severe disease. The most well-known severe form is Weil’s disease, a combination of kidney failure, liver damage (causing jaundice), and bleeding. A case report illustrating this progression described a 61-year-old man with no prior health problems who presented with fever, muscle pain, and weakness, only to be found in multiorgan failure from leptospirosis; early antibiotics and supportive care led to his recovery.5PubMed Central. Leptospirosis with acute liver injury Not every severe case ends that well.
An even more feared complication is leptospirosis-associated pulmonary hemorrhagic syndrome, or LPHS. Patients with LPHS develop coughing, chest pain, difficulty breathing, and massive bleeding into the lungs. The mortality rate for this syndrome sits at roughly 50 percent, with death typically occurring within 72 hours of symptom onset.6PubMed Central. Leptospirosis-associated pulmonary hemorrhagic syndrome: immune mechanisms, clinical manifestations, and experimental models The exact mechanism behind LPHS is still debated. Some evidence points to the bacteria directly damaging blood capillaries in the lungs, while the body’s own immune response, including a flood of inflammatory signals and activation of the complement system, may further break down the lining of blood vessels.6PubMed Central. Leptospirosis-associated pulmonary hemorrhagic syndrome: immune mechanisms, clinical manifestations, and experimental models Beyond outright hemorrhage, lung involvement can range from subtle breathing problems to full-blown acute respiratory distress syndrome.7PubMed Central. Pulmonary manifestations of leptospirosis
How It Is Diagnosed
Diagnosing leptospirosis is tricky, especially early on. The gold-standard test, called the microscopic agglutination test (MAT), works by looking for antibodies in a patient’s blood. The problem is that antibodies take time to build up, so MAT is most useful in the second week of illness and beyond. In the first few days, it can miss a lot of cases.
PCR-based tests, which look for the bacteria’s DNA directly in blood or urine, fill some of that gap. One analysis of 124 serum samples found that PCR had a sensitivity of about 62 percent in the first week of illness, while serological tests like MAT and ELISA performed better as the disease progressed; by the second week, PCR sensitivity actually dropped as the bacteria cleared from the bloodstream.8PubMed Central. Diagnostic Advances in Leptospirosis: A Comparative Analysis of Paraclinical Tests with a Focus on PCR The clinical reality is that no single test covers both early and late stages well, so doctors often combine PCR in the first few days with serological follow-up later. Rapid lateral flow assays, which work somewhat like a pregnancy test, offer quicker turnaround and are useful in resource-limited settings where sophisticated lab equipment is scarce.9South Eastern European Journal of Public Health. Emerging Trends in Leptospirosis: Advancements in Diagnosis, Treatment, and Prevention Strategies
Even in veterinary medicine, the diagnostic challenge is similar. A study comparing PCR to MAT for canine leptospirosis found that PCR on urine or blood had high specificity and positive predictive value but only moderate to low sensitivity, meaning a negative PCR result does not rule the disease out.10PubMed Central. The Evaluation of the Diagnostic Value of a PCR Assay When Compared to a Serologic Micro-Agglutination Test for Canine Leptospirosis The takeaway for both humans and animals is that paired testing, using different methods at different time points, gives the most reliable results.
Antibiotic Treatment
When caught early, leptospirosis responds well to antibiotics. Doxycycline has been used for decades and has been shown to shorten the illness by about two days while reducing fever, headache, muscle pain, and malaise. It also prevents the bacteria from being shed in urine, which matters for limiting further transmission.11PubMed. Doxycycline therapy for leptospirosis For patients who cannot tolerate doxycycline or who have more severe illness, intravenous penicillin or ceftriaxone are standard alternatives.
A systematic review and network meta-analysis comparing six antibiotics found that all of them significantly shortened the time to fever resolution compared to controls. Cefotaxime and azithromycin performed slightly better than doxycycline and penicillin in terms of how fast fever broke, though the differences were modest. However, the review found that none of these antibiotics significantly reduced mortality or shortened hospital stays.12PubMed Central. Efficacy and safety of antibiotics for treatment of leptospirosis: a systematic review and network meta-analysis That may sound discouraging, but the finding largely reflects how difficult it is to measure mortality benefits in clinical trials for a disease where the severe cases need far more than just antibiotics.
Intensive Care for Severe Cases
When leptospirosis progresses to organ failure, survival hinges on aggressive supportive care. The two organs most commonly hit are the kidneys and lungs. In a prospective study from Sri Lanka, about three-quarters of patients admitted to the ICU with severe leptospirosis needed dialysis, and roughly 57 percent required mechanical ventilation.13PubMed Central. Severe leptospirosis in the intensive care unit: single centre prospective cohort study from Sri Lanka Similar figures came from a study in tropical Australia, where about a third of ICU patients required continuous renal replacement therapy and about half needed intubation and ventilator support.14PubMed Central. Severe leptospirosis in tropical Australia: Optimising intensive care unit management to reduce mortality
Timing matters enormously. In the Australian cohort, the most common reason for starting dialysis was severe metabolic acidosis that was not responding to other treatments, and the median time from ICU admission to starting renal replacement therapy was about 10 hours.14PubMed Central. Severe leptospirosis in tropical Australia: Optimising intensive care unit management to reduce mortality Early initiation of both dialysis and protective ventilation strategies, along with careful fluid management, has been associated with better outcomes.15PubMed Central. Leptospirosis in Intensive Care Unit The difference between a well-resourced ICU and one without dialysis capacity can be the difference between life and death. This is a major reason why leptospirosis mortality disproportionately affects low-income countries.
Prevention and Prophylaxis
Avoiding contact with contaminated water is the most straightforward form of prevention, but in flood-prone tropical cities and farming communities, that advice can be nearly impossible to follow. Protective boots and gloves help for occupational exposure. Controlling rat populations around homes and workplaces makes a difference in urban settings, given the staggering volume of bacteria that even a small rat colony sheds into the environment.
Doxycycline has also been studied as a preventive drug for people at high risk. In a classic study of U.S. soldiers training in Panama, weekly doxycycline showed a protective efficacy of 95 percent against infection.16PLoS ONE. Leptospirosis Outbreak following Severe Flooding: A Rapid Assessment and Mass Prophylaxis Campaign; Guyana, January–February 2005 A non-randomized trial during urban flooding in southern Thailand found that even a single 200 mg dose of doxycycline offered a protective efficacy of about 77 percent against infection, and among people with open skin wounds, protection rose above 90 percent.17PubMed. Single dosage of doxycycline for prophylaxis against leptospiral infection and leptospirosis during urban flooding in southern Thailand: a non-randomized controlled trial During outbreaks, mass prophylaxis campaigns have been deployed, though it remains hard to separate the effect of the drug from the natural winding-down of the outbreak, receding floodwaters, and increased public awareness.16PLoS ONE. Leptospirosis Outbreak following Severe Flooding: A Rapid Assessment and Mass Prophylaxis Campaign; Guyana, January–February 2005
Vaccines exist for animals, particularly for dogs and livestock, but human vaccines have seen limited use. A key limitation is that current vaccines protect only against closely related serovars of the bacterium.18PubMed Central. Cross-protective Immunity Against Leptospirosis Elicited by a Live, Attenuated Lipopolysaccharide Mutant Because there are over 250 pathogenic serovars worldwide, a vaccine that covers one set may offer no protection against a locally circulating strain. This serovar problem is a major barrier to developing a broadly effective human vaccine, and it explains why human vaccination has only been adopted in a handful of countries, notably Cuba and China, using locally tailored formulations.
Flooding, Climate Change, and Rising Risk
Leptospirosis outbreaks and flooding go hand in hand. After the severe 2014 floods in Kelantan, Malaysia, the incidence of reported leptospirosis cases doubled compared to the pre-flood period, with hotspots concentrated in areas where garbage cleanup occurred and where people lived close to water bodies.19PubMed Central. Leptospirosis Outbreak After the 2014 Major Flooding Event in Kelantan, Malaysia: A Spatial-Temporal Analysis The pattern has repeated in Metro Manila, Taiwan, Fiji, Kerala, and Puerto Rico, among many other places.20microLife. Climate change and leptospirosis: a growing environmental and zoonotic threat
The link between climate change and leptospirosis risk is increasingly clear. As extreme weather events become more frequent and more intense, the conditions that drive outbreaks — sudden flooding, contamination of water supplies, displacement of people into overcrowded shelters — occur more often. Urban slums, where roughly a billion people live, are especially vulnerable because of inadequate drainage and sewer infrastructure. Heavy rain in these settings turns streets into open sewers, bringing rodent-contaminated water into direct contact with residents.20microLife. Climate change and leptospirosis: a growing environmental and zoonotic threat With the global urban slum population projected to double by 2050, the disease burden is expected to grow unless sanitation and drainage catch up.
Leptospirosis in Livestock and Pets
Leptospirosis is not just a human problem. In cattle and pigs, infection often causes reproductive failure: spontaneous abortions, stillbirths, premature deliveries, and the birth of weak calves. These losses can be economically devastating, especially because the infection frequently goes unnoticed. Subclinical forms of the disease in production animals are common and often underdiagnosed.21PubMed Central. Reproductive Disorders and Leptospirosis: A Case Study in a Mixed-Species Farm (Cattle and Swine) In cattle specifically, adapted leptospiral strains are more closely linked to subtle problems like subfertility and early embryonic death than to dramatic abortions, meaning the disease can silently erode herd productivity over long periods without anyone identifying the cause.22PubMed. Genital bovine leptospirosis: A new look for an old disease
Dogs are also commonly affected and, in many countries, routinely vaccinated against several leptospiral serovars. For dog owners, the practical concern is that your pet can both contract the disease from contaminated environments and potentially pass it to you through its urine if infected. Veterinary guidelines generally recommend vaccination for dogs in endemic areas, and prompt testing if a dog develops acute kidney injury, fever, or jaundice.
Long-Term Effects After Recovery
A common misconception is that once the acute infection clears, leptospirosis is done with you. For many people, that is true. But a growing body of evidence shows that some patients deal with chronic symptoms for months or even years after their initial illness. Reported post-leptospirosis complaints include recurrent migraines, chronic fatigue, depression, mood swings, and eye problems such as uveitis (inflammation inside the eye). Some patients have also been described as developing neuropsychiatric symptoms including obsessive-compulsive features and, more rarely, movement disorders.23IntechOpen. Chronic Leptospirosis in Human Patients
The mechanisms behind these chronic effects remain poorly understood, and there is debate about whether persistent bacterial presence, autoimmune responses triggered by the initial infection, or residual organ damage are to blame. What is clear is that doctors and patients should not automatically dismiss lingering fatigue, headaches, or mood changes after a confirmed case as unrelated. Follow-up care that includes screening for renal function, eye health, and mental well-being is reasonable for anyone who has had moderate or severe leptospirosis, though formal guidelines on long-term monitoring remain sparse.
The Biology Behind Its Reach
Part of what makes Leptospira so successful as a pathogen is its unusual structure. These corkscrew-shaped bacteria are extraordinarily thin and motile, able to burrow through tissue in ways that bulkier bacteria cannot. Cryo-electron tomography has revealed structural differences between disease-causing and non-pathogenic species, particularly in the outer membrane’s lipopolysaccharide (LPS) layer.24PubMed Central. Three-dimensional structures of pathogenic and saprophytic Leptospira species revealed by cryo-electron tomography Variations in the LPS layer are closely tied to serovar identity, which is why the immune system’s response to one strain does not necessarily protect against another. This structural diversity also explains the vaccine challenge mentioned earlier and is a major focus of current research aiming to identify conserved targets that could work across strains.
The organism’s ability to survive outside a host is also remarkable. Leptospira can persist for weeks in warm, slightly alkaline water and moist soil. This environmental resilience is why the disease is so tightly linked to flooding: the bacteria are already in the soil from rat urine, and floodwater acts as a delivery system, spreading the pathogen across wide areas and creating the perfect conditions for mass human exposure.